Eating a low-fat diet is known to help the heart, but now there's evidence it may also have a positive effect on certain types of breast cancer. Breast cancer patients who stick to this type of eating plan have a lower risk of their cancer returning, researchers say.

"This study may well represent the first lifestyle change—namely lowering dietary fat intake—that can have a favorable effect on breast cancer outcome," said lead researcher Rowan T. Chlebowski, MD, PhD, of the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center.

Chlebowski discussed his findings Monday at the annual meeting of the American Society for Clinical Oncology, held in Orlando. They have not yet been published in a medical journal.

For their study, he and his colleagues recruited nearly 2,500 postmenopausal women (ages 48-79) who had been treated for early-stage breast cancer with surgery and standard follow-up care (such as radiation, tamoxifen, or chemotherapy). Of this group, they randomly assigned 975 women to eat a low-fat diet—about 33 grams of fat daily.

The researchers didn't advocate any particular brand of diet, just as long as it was low in fat intake. The women in their study had access to a registered dietician, who gave them pointers on how to cut some fat out of their regular diets. For instance, women who usually ate a sweet roll for breakfast were advised to substitute cereal instead. Those who typically snacked on high-fat products like chips were asked to munch on plain popcorn instead.

The remaining 1,462 women followed their normal diet, with only brief counseling about getting adequate nutrition. On average, these women consumed about 51 grams of fat each day.

After about 5 years of follow-up, the low-fat diet had helped some of the women—those with estrogen-negative (ER-negative) tumors—lower their risk of recurrence by 42%. ER-negative tumors do not have estrogen receptors on them and therefore don't respond to hormonal treatments like tamoxifen or aromatase inhibitors. That makes them more difficult to treat than hormone-sensitive tumors (also called ER-positive tumors).

The diet did not significantly lower recurrence risk in women with ER-positive tumors.

The idea that fat intake might affect breast cancer grew out of the knowledge that fat affects estrogen production, and estrogen fuels breast cancer. But the low-fat diet only seemed to impact women with estrogen-negative tumors, suggesting other mechanisms play a role in breast cancer, Chlebowski said.

"One big outgrowth of this will be . . . getting people to look for new targets for breast cancer therapy," he said.

About one-third of women past menopause develop ER-negative breast tumors, said Len Lichtenfeld, MD, deputy chief medical officer for the American Cancer Society. For these women, he said, the new findings are exciting, but still preliminary.

"We've been hearing about the potential of low-fat diets to affect cancer for decades," Lichtenfeld said. "Until now, we had no evidence this had a significant impact. Now we have some, but it really has to be confirmed in larger trials."

Chlebowski agreed.

"If these results are confirmed in additional trials, reduction of dietary fat intake could be considered part of the management of breast cancer in postmenopausal women," he said. "Patients would then have an additional option within their control for reducing the risk of breast cancer."

But women don't need to wait to start eating more healthfully.

"There's never harm in following a balanced low-fat diet," Lichtenfeld said. "It's certainly appropriate for [a postmenopausal woman with ER-negative tumors] to consider it after speaking with her physician. However, until this study has been confirmed by others, we can't say with absolute certainty that this will be beneficial for her breast cancer."

Additionally, Chlebowski warned, women should not expect any kind of diet plan, low-fat or otherwise, to take the place of traditional breast cancer treatment with surgery and medications.